The term "local network" denotes networks which connect users scattered over distances ranging from a few hundred metres to some ten kilometers. These networks, which generally consist of a common transmission medium connecting all users, were first designed to connect all processing resources of a firm (computer local networks); then, they were used for both data and speech transmission (local area communication networks) and today they are developing towards an integration in a unique network of all communication services of a firm (integrated services communication systems). Then, reliability and availability of this unique network are essential requirements.
Reliability aspects refer to both access protocol and network topology. In no case electric or mechanical failures should cause a general service breakdown lasting more than a few ten milliseconds.
As for the access protocol, the use is towards distributed techniques, in a network with ring or bus configuration, which assure the right operation even in the presence of failed stations.
For the topology, a ring structure presents two main reliability problems. First, information is to be suppressed at the end of its path over the ring, e.g. by the transmitting station or a centralized device.
A centralized device eases the check of the occurred information suppression and makes it possible to recover the failure situations due to misfunctioning of the transmitting or receiving station, even in the case of a failure of the station which should eliminate the information, but it has an unsatisfactory reliability. In fact, non-catastrophic failures of the centralized device could lead to the systematic introduction of errors in the packets passing in the physical medium, while serious failures compromising its operation should cause activity interruption. Then, the duplication of this device would be unavoidable, but this would be of difficult solution due to the complex functions carried out and to the fact that the presence of two devices would require a continuous dialogue between them in order to establish the active device: in fact, the two devices should be placed in different points of the network, in order to hinder that a failure in a very limited area of the building isolates both devices, thus making the duplication useless.
Furthermore, the use of token passing protocols, commonly employed in ring networks, requires handling mechanisms such as to avoid that failures and malfunctionnings bring about a systematic token loss or duplication; in fact, such events will imply the network re-initialisation with a consequent loss in information and degradation of the quality of the service offered.
A bus structure is more reliable as, being open the common transmission line, information is automatically suppressed at the line end and a specific suppression procedure is unnecessary.
Furthermore, using the bus, the station couplers can be implemented with passive components, as the explicit signal suppression is no more necessary with a consequent reliability advantage. Still further, if future developments of communications should be considered, and hence the network should be used not only for conventional speech and data services, but also for new services such as high resolution graphic services, CAD (computer aided design) or CAM (computer aided manufacturing), videoconference with moving images, remote control of production processes, high resolution diffusive video services, etc., then bus structure well fits with the handling of hybrid switching technique which allows an effective management both of communications which present continuity characteristics and/or require synchronism maintaining, and of communications which do not present these requirements, and with the use of ordered access protocols.
This performance presents also advantages from reliability viewpoint; in particular, by an ordered access protocol, a failure which hinders transmission by a station implies the non use of the access right which is automatically transferred to the next station; therefore, a failure of a station does not affect the other stations, being interpreted as a transmission giving up. Furthermore, the presence of a cyclic frame implies no error memory, i.e. a temporary disturb which affects the correct transmission of information by one or more stations in a frame does not affect the information transmitted in the next frame.
Notwithstanding this intrinsic reliability, a bus structure with ordered access and hybrid frame transmission is affected by failures which cause a line interruption between two stations or, in the case of active networks (i.e. with signal regeneration in all or at least some stations), by the failures of station transceivers. Such failures could put out of service the whole network, as generally connection paths alternative to the common transmission line do not exist between the different network points.
Theoretically, a line failure could be remedied by a line duplication. Nevertheless, an active network requires also the duplication of the devices which, in the stations with signal regeneration, give access to the network (in particular at least the devices which manage the physical protocol level): costs are then higher. Furthermore, the two lines obtained with the duplication should be installed in different ducts, otherwise the same event could interrupt both lines: in this case, the two lines may have different lengths and could cause different propagation delays and then a complication of the algorithm handling the communications. A further source of complexity is the necessity of an algorithm for the definition of the "hot" line (line actually used for data transfer) and stand-by line.
An alternative solution is the insertion of switches along the network which automatically operate in the case of a failure of the network section protected to by-pass the section failed. For example, in the case of an active network, the switches could be placed upstream and downstream a station and they could by-pass the station in the presence of failures: thus, considering the line sections near the station as secondary sections and the remaining line as main section and placing the latter in highly protected ducts so as to limit the probability of mechanical failures, also the transmission line is partially protected. The main drawbacks of this solution are that the switches require quite long command times and the sections activated by the switches can greatly change the connection length and then originate different propagation times and higher complexity of transmission-reception devices.
To solve these problems, a reconfigurable local network has been proposed, i.e. a network that in the case of a failure in the line or in one or more stations, take such a topology as to connect all or most operating stations, isolating the failed station(s) or line section(s).
Obviously, reconfiguration should be as quick as possible, so as to allow a rapid service restoration, even if with reduced performance; furthermore, the devices which handle the reconfiguration should also allow a quick normal activity resumption, once the failure was repaired.
An example of reconfigurable network is described in the article "Performance Evaluation of Reliability Improvement Techniques for Single Loop Communications Systems" by P. Zafiropulo, IEEE Transactions sactions on Communications, vol. Com-22, No 6, June 1974.
This article describes a network with a duplicate transmission line, consisting of two rings where only one is active under regular operation conditions of the network: the stations and, at prefixed intervals, reconfiguration units are placed along the line. These units essentially consist of an input and an output port for each ring. Under regular operation, reconfiguration units connect the input and the output port of the same ring: in the case of a failure in a station or a line section, in the unit upstream and downstream the failure, a connection is established between the input port of a ring and the output port of the other ring at the same side, so as to isolate the failed section. The article describes in detail neither the structure of the reconfiguration units nor the ways and times of reconfiguration and regular service resumption.
This known structure presents some disadvantages.
The first concerns the ring structure with central controller: a failure which isolates the section comprising the controller puts out of service the whole network without remedy; hence, a section exists which can undergo catastrophic failures. Also, due to the use of a token passing protocol, the operations of network reconfiguration and recovery are more delicate because of the necessity of not loosing or not duplicating the token. Furthermore, the duplication of transmission medium and transceivers is necessary with respect to the configuration sufficient for the network unprotected against failures.